Currently the Release-11 (Rel-11) work item ‘MIMO with 64QAM for HSUPA’, RP-111642, by Nokia Siemens Networks, is standardized in Third Generation Partnership Project (3GPP). The objective of this work item is to specify the support of uplink 2×2 Multiple-Input-Multiple-Output (MIMO) with 64-Quadrature Amplitude Modulation (64QAM) as an additional feature for High Speed Uplink Packet Access (HSUPA) in Frequency Division Duplex (FDD). Uplink (UL) MIMO allows the user equipment (UE) to simultaneously transmit up to two Media Access (MAC)-i Packet Data Units (PDUs), or transports blocks (TBs), in the same Transmission Time Interval (TTI) on orthogonal beams, (virtual antennas). Thus, there may be two streams in each TTI.
Power control is fundamental in an HSUPA system in order to combat a so called near-far problem, and to hamper the effect of channel fading. In general, the near-far problem relates to a condition in which a receiver captures a strong signal and thereby makes it impossible for the receiver to detect a weaker signal. Provided that the dynamic range of the transmit power of the user equipment is large enough, and that the power update is faster than the fading, it follows that the channel becomes an essentially non-fading channel. The basic operation of fast power control, also referred to as inner-loop power control (ILPC), is as follows. The estimated received Signal-to-Interference Ratio (SIR) is compared to the target SIR value. If the measured SIR is higher than the target SIR, the base station will command the user equipment to lower its power, and vice versa. The SIR target is controlled by the outer-loop power control (OLPC) mechanism in order to achieve a certain quality target for the data (E-DCH Dedicated Physical Data Channel, where E-DCH stands for Enhanced Dedicated Channel, or abbreviated as E-DPDCH). In one example, the certain quality target is Block-Error-Rate (BLER) or Number of Hybrid Automatic Repeat request (HARQ) Retransmissions (NHR). Hence, if the quality is lower than the target quality, the SIR target is increased and vice versa. Consequently, the OLPC is one means to control the quality of data (E-DPDCH). The OLPC is located in the Radio Network Controller (RNC), and the exact algorithm is implementation specific and hence not standardized. For Dedicated Channel (DCH) traffic, the OLPC typically sets the SIR target to fulfil a certain BLER target. For Enhanced-Uplink (EUL) traffic, on the other hand, the BLER is a poor measure to base the SIR target on. One reason is that the HARQ functionality is located in the NodeB and another reason is that the BLER that is visible to the RNC will in most cases be close to zero. That is to say, a BLER that is close to zero provides no or little information about data quality. A better measure of data quality for EUL traffic is therefore the number of transmissions required for successful decoding, i.e. a specific number of transmission attempts for successful decoding is targeted.
In soft/softer-handover, the UE will get ILPC commands from several nodes. In this case the UE will follow a “DOWN-before-UP” principle, i.e. it is enough that one node indicate a DOWN command for the UE to lower its power. Also, due to soft handover (SHO), the data quality from several nodes may influence the SIR target, and therefore the OLPC mechanism is located in the RNC. Several nodes may forward information about the data quality to the RNC, which uses this information to set the appropriate SIR target. Typically, the nodes report the number of transmission attempts required to successfully decode the data packet (or if data failed to be decode after the maximum number of transmission attempts). Since the OLPC resides in the RNC, it is rather slow and may be subject to delayed information.
HARQ is used to overcome transmission errors that can not be corrected using forward error correcting codes (channel codes). At the transmitter side a Cyclic Redundancy Check (CRC) code is appended to each transport block. The decoder at the receiver side can then use this CRC code to validate the received and decoded transport block. If the CRC checks (true), the transport block is considered to be successfully received, otherwise the transport block is considered to be subject to errors and needs to be retransmitted. If the CRC checks, i.e. is correct, a positive acknowledgment (ACK) is sent by the receiver to the transmitter, whereas if the CRC fails, i.e. is wrong, a Negative Acknowledgment (NACK) is conveyed to the transmitter. In soft/softer handover, the UE will get acknowledgment messages from several nodes, and in this case it is enough that one node sends an ACK, to consider the packet successfully received. Once a node has successfully decoded a packet, that node will forward the packet to the RNC. In addition to the decoded packet, the node will forward information about the number of transmission attempts needed to decode the packet. This information is then used by the RNC to set the appropriate SIR target.
In the current frame protocol, as specified in “UTRAN Iub/Iur interface user plane protocol for DCH data streams”, 3GPP TS (Technical Specification) 25.427 (v11.4.0), for each subframe of the Enhanced Dedicated Channel (E-DCH) Type 1 and Type 2 frame, there exist an Iub information field for signalling the number of HARQ retransmissions from NodeB to a RNC, such as a serving RNC (SRNC) (See 25.427; Section 6.2.4.11). When the TTI length is 2 ms, one Iub Frame can contain up to 5 subframes; when the TTI length is 10 ms, one Iub Frame can contain 1 subframe.
Number of HARQ Retransmissions (NHR) as Currently Defined in 25.427:
Description: Indicates the number of HARQ retransmissions used for successful decoding of the payload, or in case of HARQ decoding failure the number of HARQ retransmissions that were used at the time when the HARQ decoding failure was detected. The value 13 indicates that the actual number of retransmissions is inappropriate as input to the outer loop power control. The value 15 indicates that the Node B could not calculate the number of HARQ retransmissions.
Value Range: {0-15}
Value {12}: Used for indicating that the number of HARQ retransmissions was 12 or higher.
Value {13}: Used for indicating that the number of HARQ retransmissions shall not be used by the outer loop power control.
Values {14}: Reserved in this user plane revision. Shall be ignored by the receiver.
Value {15}: Used for indicating that the number of HARQ retransmissions is unknown.
Field length: 4 bits.
With the definition directly above, there is a risk that the OLPC, as described somewhat further above, may fail when user equipments operate in UL MIMO.